This invention relates to an apparatus and method for varying the timing and duration of both inlet and exhaust poppet valves of a combustion cylinder of an internal combustion engine.
Due to fuel specifications the majority of post 1976 motor vehicle internal combustion engines require retarded intake valve timing to lower the physical compression to prevent xe2x80x9cpreignitionxe2x80x9d. The problem of preignition typically occurs is when the heat and compression in a combustion cylinder reaches a point in front of the burning air/fuel mixture such that an explosion takes place that is undesired to other internal components. In recent years the octane level in petroleum has been lowered which has compounded the preignition problem.
Two disadvantages associated with retarded valve timing to avoid preignition are xe2x80x9cgas refluxxe2x80x9d and a decline in low speed torque. Gas reflux is a problem that occurs at low rotational speeds of an engine. At low rotational speeds the pressure in the combustion chamber is higher than that of the intake tract (inlet manifold) which interrupts the gas flow inertia xe2x80x9cramming effectxe2x80x9d, In order to overcome this problem engine manufacturers have had to increase the idle speed to ensure a smooth engine idle. This in turn creates a problem with an increase in fuel consumption and exhaust emissions and engine wear.
Manufacturers have developed camshafts with the retarded intake valve timing to reduce physical compression in order control the problem of preignition. Such known camshafts have compromised engine performance.
During the operation of a combustion cylinder of an internal combustion engine, xe2x80x9cdwellxe2x80x9d occurs between the point the camshaft actuates the valve to open and the point it allows the valve to close. xe2x80x9cValve overlap periodxe2x80x9d, also known as xe2x80x9cvalve overlap anglexe2x80x9d is the sequence at which both inlet and exhaust valves are open during a rotational cycle of the engine. xe2x80x9cDurationxe2x80x9d is the period the valve remains open. High performance engines require a large xe2x80x9cdwellxe2x80x9d and xe2x80x9cvalve overlap periodxe2x80x9d to allow more time for the piston to draw in a significant amount of air and fuel at high crankshaft speeds, but at lower speeds xe2x80x9cgas refluxxe2x80x9d retards the performance of the engine. Low speed torque requires a reduced dwell and valve overlap period to lower gas reflux. It also requires that the intake valve closure occurs early on the compression stroke of the cylinder to increase physical compression.
Several automotive manufacturers have developed variable valve timing mechanisms without variable xe2x80x9cdurationxe2x80x9d. A problem associated with variable valve timing without variable duration, is that as xe2x80x9cdwellxe2x80x9d remains constant, the point at which the valve opens equally effects the point of valve closing. This constant xe2x80x9cdwellxe2x80x9d effectively disadvantages some of the benefits gained from advancing or retarding the valve timing. This effect becomes more pronounced for an engine with one camshaft that actuates both the intake and exhaust valves.
Australian Patent No. 695596 (Bentley) discloses an apparatus for operating poppet valves, which allows for variability of the dwell, timing, duration and overlap. The apparatus comprises a means for rotating two camshafts, an inlet camshaft and an exhaust camshaft. The valve timing is varied by two sun gear sets each of which is associated with a respective camshaft and both of which are operably connected to a servomechanism. The servomechanism acts on an annulus operably connected to the planetary gear assembly. A disadvantage of this device is that the sun gear assemblies and servomechanism are located between the crankshaft and belt (or chain) hub(s), and operably engaged with an idler gear, thereby adding considerable complexity and expense to the apparatus.
It is an object of the invention to ameliorate the operation of an internal combustion engine.
In a first aspect the present invention consists of a valve actuating apparatus for at least two valves of an internal combustion engine, said apparatus comprising a drive means for rotating at least two camshafts each of which actuate a rocker assembly which in turn actuates said valves, said drive means comprising a drive gear to operably rotated by the engine crankshaft and operably engaged with two driven gears each of which is slidably mounted on a respective one of said at least two camshafts, wherein each of said driven gears are adapted to be slidably moved along their respective camshafts by a servo means.
In one embodiment said drive gear and said two driven gears are helical gears, and said slidable movement of said two driven gears along their respective camshafts is parallel to their respective axes of rotation.
In a second aspect the present invention consists of a valve actuating apparatus for at least two valves of an internal combustion engine, said apparatus comprising a drive means for rotating at least two camshafts each of which actuate a rocker assembly which in turn actuates said valves, said drive means comprising a helical drive gear operably rotated by the engine crankshaft and operably engaged with two helical driven gears each of which is slidably mounted on a respective one of said at least two camshafts, wherein each of said helical driven gears are adapted to be slidably moved along their respective camshafts by a servo means.
Preferably the servo means is operably controlled by an electronic control unit. Preferably said electronic control unit is operably connected to a number of engine sensor units which monitor engine parameters such as manifold air pressure, engine temperature, crankshaft angle(rotational position), camshaft angle(rotational position) and ignition.
Preferably the servo means comprises at least two servo units each of which is associated with a respective one of said helical driven gears.
Preferably each of said servo units is adapted to slidably move said helical driven gears independently of each other.
Preferably the two helical driven gears are each slidably mounted at or near respective ends of the camshafts by means of a spline.
In a third aspect the present invention consists of an internal combustion engine having at least one actuating apparatus for actuating at least two valves, said apparatus comprising a drive means for rotating at least two camshafts each of which actuate a rocker assembly which in turn actuates said valve, said drive means comprising a helical drive gear operably rotated by the engine crankshaft and operably engaged with two helical driven gears each of which is slidably mounted on a respective one of said at least two camshafts, wherein each of said helical driven gears are adapted to be slidably moved along their respective camshafts by a servo means.
The present invention preferably allows for variable valve timing and duration by changing one or more camshaft actuations of the valve to improve the torque, power output, fuel economy and exhaust emissions of the engine.
Preferably the present invention provides an improvement in xe2x80x9cvolumetric efficiencyxe2x80x9d by control of the valve operation. Volumetric efficiency is the measurement in percentage of the air/fuel mixture drawn into the combustion chamber during the intake cycle.